This invention relates to zoom lenses having focusing provision made at the front component thereof, and, more particularly, to a method of controlling spherical aberration to be stabilized throughout the entire focusing range as extended toward shorter object distances.
Zoom lenses having their shortest focal lengths longer than 60 mm, or less to 55 mm. are commonly known as "telephoto type zoom lenses", examples of which are disclosed in U.S. Pat. Nos. 3,336,094 and 3,615,125. Of the many known telephoto type zoom lenses, there are some zoom lenses having focusing provisions for permitting close-up adjustment. However, about the shortest object distance they can achieve is on the order of 1.8 to 2 meters, and this can be done only by tolerating considerable image aberrations.
The prior art efforts to correct aberrations have generally been directed toward an object plane at infinity. With such lens systems, however, it is frequently necessary to focus down to shorter object distances but usually with some loss in aberration correction. Particularly when focusing is achieved by imparting an independent movement to one member of the lens system, it is found that aberrations are deteriorated to a large extents. This aberrational problem becomes serious when the focusing range is extended toward object distances shorter than 2 meters. For example, when the focusing provision is made at the front lens component as is usually employed in lens systems of the zoom type, it is very difficult to achieve good stabilization of correction of spherical aberration, coma, astigmatism and distortion throughout the focusing range particularly at the telephoto settings, although these aberrations are not seriously affected in the wide angle settings except for a change in sign of distortion from negative to positive.
In the past, variation of aberrations produced by a telephoto type zoom lens having a front component movable for focusing has been prevented by correcting the front component itself to a sufficient degree for objects at infinity by use of the ordinary lens design method. In this case, however, the number of lens elements necessary is increased with a corresponding increase in the weight and bulk of the forward part of the complete system and also in the physical length thereof to such a large extent that they cannot be kept within easily manageable proportions. It is also known to find the principal laws of correction of the aberrations for object points near the center of a distance between infinity and close-up. This method does, however, not enable the provision of a telephoto type zoom lens substantially free from variation with focusing of the aberrations, but only to distribute a certain amount of aberrations over the entire focusing range as well as over the zooming range.